In computer system's logical partitioning (or virtualization) technologies, control software called hypervisor or virtual machine monitor (VMM), running in privileged mode, controls the physical hardware of the computer system.
The hypervisor exercises control so as to logically divide the hardware resources of the physical machine such as CPUs, memories, and I/O devices to configure a plurality of logical partitions on the computer system. In each of the logical partitions on the computer system, the hardware resources distributed can be used to run a virtual machine (including an operating system).
With the logical partitioning technologies, the I/O devices are controlled by a privileged device driver in the hypervisor in a centralized fashion. The device drivers included in each operating system therefore need to access the I/O devices through the privileged device driver when controlling the I/O devices. The presence of the processing through the privileged device driver causes a large overhead, so that the logical partitioning (or virtualization) environment has sometimes failed to provide satisfactory performance.
In view of the foregoing, there is now adopted a method in which the hypervisor only performs resource allocation management on the I/O devices, and the I/O devices are directly controlled by the device drivers that are included in the operating systems on the respective virtual machines (pass-through I/O method). The pass-through I/O method can be used to solve the foregoing problem that the I/O devices need to be accessed through the privileged device driver for I/O device control.
Meanwhile, there are technologies in which a plurality of physical machines are prepared and virtual machines are dynamically migrated (moved) within an identical physical server or between different physical servers. For example, a virtual machine can be migrated from a physical machine in a high load condition to one in a low load condition so that the physical machines are leveled out in the load condition (for example, see paragraph [0017] of PTL 1, paragraphs [0019] and [0020] of PTL 2, and so on).
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    {PTL 1} JP-A-2005-327279    {PTL 2} JP-A-2008-217302